The other day, Googler Skylar said he was unable to find any issue. He said in the Google Web Search Help thread:

Thanks everyone for providing information about your computers and browsers. I’ve been passing these details along to the rest of the team to check on SafeSearch filtering. We're currently unable to find errors with SafeSearch filtering. It is likely that a corrupt cookie, a third-party add-on, or an anti-virus program could be interfering with your preferences. I recommend giving the "Preferences aren't sticking" help article a try to make your preferences stick. In the meantime, I understand that it is frustrating when your preferences don't stick, and I'm sorry for any inconvenience you're experiencing.

But this does not explain why hundreds of searchers are still having this issue.